Wireless telephones may be employed in many different practical applications. For example, they occur in the home in order to circumvent the restriction involved in a cord to the telephone receiver. They also occur within industry and in military applications. In the two latter cases, it is a common occurrence that several people each have their communication unit or handset which are all hooked up to the same base unit, which makes for conversations between those persons having a handset.
One standard employed for wireless telecommunication is DECT (Digital Enhanced Cordless Telecommunications). This standard was set up in 1992 and since then has been revised on numerous occasions. The range of this system is approx. 50 m indoors and approx. 300 m outdoors. The DECT system operates with ten ms long time windows, which in turn are divided into 24 time slots, i.e. shorter, well defined time intervals. Half of these may be utilised for transmitting and half for receiving. The system also utilises ten different frequencies, and together this may be said to build up a system with a maximum of 120 channels. In practice, the different frequencies are employed for a “frequency shopping” which is determined by the system, so that those frequencies that momentarily enjoy the best reception conditions are used first. As a result, in reality there are twelve different channels, which implies that hitherto it has been possible to hook-up a maximum of twelve handsets to one and the same base station, which functions as the nerve centre of the system. The base unit checks in rapid sequence each one of the channels and retransmits possible signals to all of the other channels to which a handset is hooked up. The existing system operates per se satisfactorily, and is well-rehearsed to be integrated in, for example, hearing protection and is ideally utilised for communication within small groups. The present system of operation by utilising the standard however entails certain drawbacks. The modus operandi requires considerable energy, since the base station transmits identical signals up to twelve times. This may be a drawback which is particularly severe when the base station is battery powered.
Since a maximum of twelve handsets can be used together with each base station, the size of the groups that can communicate with one another freely within the group is limited.